It is known that fluorine can be exchanged for chlorine, bromine or iodine bonded to phosphorus by reaction of the halophosphorus compound with a metal fluoride. The transhalogenation reaction is quite slow and difficult to push to completion. It is sometimes desirable to replace chlorine, bromine, or iodine bonded to phosphorus with a fluorine atom. For example, Burton U. S. Serial No. 020,023 filed Feb. 27, 1987 describes a family of hydrocarbyl fluorophosphites that are very effective stabilizers in polyolefins, especially in combination with phenolic antioxidants, and are also hydrolytically stable. These compounds are made by first forming a hydrocarbyl chlorophosphite by reaction of an appropriate aliphatic or aromatic hydroxy compound with PCl.sub.3 to form a mono or dichlorophosphite and then transhalogenating the chlorine atom with fluorine by reaction with a metal fluoride such as potassium fluoride.
L. P. J. Burton and M. S. Ao, in U.S. Serial No. 110,198 filed Oct. 19, 1987, disclose that the transhalogenation of a chlorine, bromine or iodine atom bonded directly to phosphorus by reaction with a fluoride salt can be sharply promoted by including in the reaction mixture a hydrogen halide salt of a pyridine-type compound such as pyridine hydrochloride. While this constitutes an important contribution to the art by decreasing the amount of time required to complete the transhalogenation reaction, it is necessary to remove the promotor from the product of reaction at the completion of the reaction by introducing ammonia into the reaction mixture to convert the pyridine hydrochloride catalyst to ammonium chloride and separate it from the reaction mixture as a precipitate. At the plant-size scale, this is a difficult and expensive procedure and also creates a waste disposal problem since the precipitate typically contains solvent used in the process which must be removed from the filter cake before disposal. Further, pyridine is freed from the catalytic complex by the introduction of ammonia into the reaction mixture which must be separated from the reaction product by distillation and recycled for reuse. This also adds to the cost of production. Also, the pyridine-HCl complex is corrosive of the metal equipment in which the product is made.
In pending application Serial No. 227,231 filed by K. A. Keblys, M. S. Ao and L. P. J. Burton there is disclosed a method for exchanging a halogen bonded to a phosphorus atom with fluorine by reacting a phosphorus compound having a halogen selected from chlorine, bromine or iodine bonded directly to phosphorus with a hydrogen fluoride salt of a pyridine-type compound such as pyridine hydrofluoride in an inert solvent. This method also constitutes an important contribution to the art by providing for shortened transhalogenation reaction times and avoiding the use of expensive metal fluoride reactants such as potassium fluoride inherent in the practice of the process disclosed in aforediscussed application Serial No. 110,198. However, a disadvantage of this method is that pyridine hydrochloride is formed as a by-product from the reaction of a portion of th pyridine hydrofluoride with the halophosphorus starting material, which, as previously discussed, requires removal from the product of reaction at the completion of the reaction by introducing ammonia into the reaction mixture to convert the pyridine-hydrochloride to ammonium chloride and thereafter separating it from the reaction mixture as a precipitate, which, as noted previously, results in economic penalties. Thus, a need exists for an improved method.